Physics

ABSTRACT
Mirror-image asymmetric molecules, i.e., chiral isomers or enantiomers,
are classically considered as chemically identical. Recent studies,
however, have indicated that parity violation by the nuclear weak force
induces a tiny energy difference between chiral isomers. Upon
combination with a massive amplification process, expansion of this
difference to a detectable macroscopic level may be achieved. Yet,
experimental tests of this possibility, where one enantiomer is
compared to the other in solution, are hampered by the possible
presence of undetectable impurities. In this study we have overcome
this problem by comparing structural and dynamic features of synthetic
D- and L-polyglutamic acid and polylysine molecules each of 24
identical residues. In these water-soluble polypeptides helix formation
is an intramolecular autocatalytic process amplified by each turn,
which is actually unaffected by low level of putative impurities in the
solvent. The helix and random coil configurations and their transition
were determined in this study by circular dichroism (CD) and isothermal
titration calorimetry (ITC) in water and deuterium oxide. Distinct
differences in structure and transition energies between the
enantiomeric polypeptides were detected by both CD and ITC when
dissolved in water. Intriguingly, these differences were by and large
abolished in deuterium oxide. Our findings suggest that deviation from
physical invariance between the D- and L-polyamino acids is induced in
part by different hydration in water which is eliminated in deuterium
oxide. Based on the recent findings by Tikhonov and Volkov (V. I.
Tikhonov and A. A. Volkov, Science 2002, 296, 2363) we suggest that
ortho-H(2)O, which constitutes 75% of bulk H(2)O, has a preferential
affinity to L-enantiomers. Differential hydration of enantiomers may
have played a role in the selection of L-amino acids by early forms of
life.